88 UPLIFT OF STRATA AT CAJON PASS. 



the Great Basia, wliicli, like a curtain, stretch across and close up the natural hiatus in the 

 upheave of primary rock. Similarly is it with San Grorgoiio pass ; like the Cajon, its direction 

 is south a little east; and, instead of crossing the mountain mass, it takes a low valley between 

 San Jacinto and San Gorgoiio mountains, and leads into the Colorado desert. 



The elevation of these passes varies considerably. Thus the altitude of each summit above 

 the sea is, in feet — 



Of Pass San Francisquito, 3,437. 



Of New pass, 3,164. 



Of Cajon pass, 4,676. 



Of San Gorgono, 2,808. 



On reaching the summit of any of these passes the strata of the basin and desert (sandstones 

 and conglomerates) are immediately encountered ; showing not only the increased elevation of 

 the Great Basin above the Pacific slope, but proving the fact of the communication of the waters 

 on both sides of the sierra during the period of the deposition of the coarse sandstone (Eocene) 

 found forming the rim of the basin. 



In fact, these strata reach the summit of the range in many places, as in Pass San Francis- 

 quito, and within a few hundred feet of it, at Cajon pass, and still less at New pass. This is so 

 remarkably the case at the Cajon pass as to form the peculiarity whence it has derived its name. 

 In ascending this pass from San Bernardino valley, at the level of 2,000 feet, the pass com- 

 mences through primary rocks which rise up to no great altitude in the vicinity of the pass ; 

 further up dislocated sandstones and alluvial beds, occupying small basins, are met until the 

 head of the creek is reached. The primary rocks now no longer appear, yet the summit of the 

 pass has not been reached. The valley, if it may be so termed, in which the creek heads, 

 faces the south with its only opening, "hemmed in on every other side by loose and not very 

 well defined stratified sandstone and conglomerate, which rise almost abruptly 100 or 800 feet 

 above the creek bed. The edges of the strata, which are almost horizontal, form the walls of 

 this cajon, or box, and once no doubt occupied the whole pass to the granitic rim at its southern 

 edge ; the fluviatile action during the modern period having worn out the present bed and 

 valley in this intramontane locality. The vast amount of rounded driftstones carried down 

 into the San Bernardino valley attest how much can be done by a single small stream acting 

 during a prolonged period. When it is stated that the Eocene desert beds are found rising to 

 the level of all the mountain passes, it is not inferred that the waters on either side of the j^rimitive 

 axis were of the depth indicated by the altitude of the passes ; there is abundant evidence of 

 the reverse. One circumstance alone would render this unlikely. These desert strata at the 

 Cajon are deposited unconformably upon a granite which drops very abruptly, and upon which 

 originally it never could have received any marked inclination at the time of deposit. The 

 slope is about 7° toward the basin, which is due to the elevation of the Kikal Mungo and San 

 Bernardino hills, which, in their final uplift, carried these (once) horizontal beds along witli 

 them, and thus gave them the dip they at present possess. That this dip is due to this cause 

 is apparent by examining the same strata at San Francisquito and about Lake Elizabeth, 

 where they appear almost perfectly horizontal, and perhaps have a dip less than 3'^ towards 

 the basin. In other words, the difference of elevation between New pass and Cajon pass (1,500 

 feet) would account for the variable dip of the strata at the two points. In this case, it is sup- 

 posed that the whole chain was of a pretty uniform height throughout its whole extent at the 

 time of deposition of these beds, since when, the whole has been elevated slowly — not only the 



